1. Field of the Invention
The present invention generally relates to communication systems, and more particularly to a system and method for encoding DSL information streams having differing latencies.
2. Discussion of the Related Art
In recent years telecommunication systems have expanded from traditional POTS communications to include high-speed data communications as well. As is known, POTS communications include not only the transmission of voice information, but also PSTN (public switched telephone network) modem information, control signals, and other information that is transmitted in the POTS bandwidth, which extends from approximately 300 hertz to approximately 3.4 kilohertz.
New, high-speed data communications provided over digital subscriber lines (DSL), such as Asymmetric Digital Subscriber Line (ADSL), Rate Adaptive Digital Subscriber Line (RADSL), High-Speed Digital Subscriber Line (HDSL), etc. (more broadly denoted as xDSL) provide for high-speed data transmissions, as is commonly used in communicating over the Internet. As is known, the bandwidth for xDSL transmissions is generally defined by a lower cutoff frequency of approximately 30 kilohertz, and a higher cutoff frequency which varies depending upon the particular technology. Since the POTS and xDSL signals are defined by isolated frequency bands, both signals may be transmitted over the same two-wire loop.
Indeed, twisted pair public telephone lines are increasingly being used to carry relatively high-speed signals instead of, or in addition to, telephone signals. Examples of such signals are ADSL (asymmetric digital subscriber line), HDSL (High Density Subscriber Line, T1 (1.544 Mb/s), and ISDN signals. There is a growing demand for increasing use of telephone lines for high speed remote access to computer networks, and there have been various proposals to address this demand, including using voice over data systems to communicate signals via telephone lines at frequencies above the voice-band.
As is known, different applications often demand (or at least lend themselves to) different latency requirements. For example, applications of pure data transfer are often not sensitive to latency delays, while real-time voice communications are sensitive to latency delays. As is also known, to accommodate maximum flexibility for providers and end users of ADSL services, forward error correction (FEC) may be selectively applied to the composite data streams to, or from, the central office ADSL modem. This permits FEC to be included or excluded on a data service by data service basis within the composite data stream.
As an example of the mixed requirements for FEC in an ADSL service, consider transmitting a one-way data stream from the central office to a remote unit. The end user may require high reliability on the one-way channel because the channel may contain highly compressed digital data with no possibility for requesting retransmission. For this application, FEC is highly desired. On the other hand, voice services and duplex data services with their own embedded protocols may require minimum latency. As noted above, in real-time voice communication applications, latency delays are undesirable, while small transmission error may be tolerated (manifested as noise, which can be effectively filtered by the listener). Thus, in such an application, FEC may be optional.
FEC involves the addition of redundant information to the data to be transferred. The data to be transferred, along with the redundant data when added together, form what are commonly known as codewords. FEC in ADSL employs Reed-Solomon codes based on symbols of 8 bits to a byte. FEC in ADSL is rate adaptable, providing for various interleave depths and codeword lengths to support a range of data rates while maintaining constant interleave latency. An enhancement to FEC involves shuffling or interleaving the encoded data prior to transmission, then unshuffling or deinterleaving the data received at the remote DSL modem. Interleaving ensures that bursts of error noise during data transmission do not adversely affect any individual codeword in the transmission. If noise affects a particular frame of data, only a minimum number of bytes of any particular codeword will be affected as the individual codewords are distributed across multiple frames.
The combination of Reed-Solomon encoding with data interleaving is highly effective at correcting errors caused by impulse noise in the service subscriber's local loop. In convolutional interleaving, after writing a byte into interleave memory, a previously written byte is typically read from the same memory.
Standard T1.413, Interface between Networks and Customer Installation—ADSL Metallic Interface provides for convolutional interleaving/deinterleaving along with Reed-Solomon coding as part of forward error correction (FEC). The standard provides an effective method for dealing with burst error channels in modem telecommunication systems. In DMT systems, two latency channels are supported: interleave data and fast data (without interleaving). Convolutional interleaving/deinterleaving is typically implemented by processing the Reed-Solomon encoded digital data sequence through a linear finite state shift register. In high bit rate applications like DMT, a random access memory (RAM) device may be used as the data storage means. Convolutional interleaving/deinterleaving is computation intensive. In software approaches that use a single address pointer and several modulo and addition operations to update the address pointer, system level concurrency and performance is adversely affected. Conversely, hardware approaches that utilize multiple pointers for interleaving/deinterleaving operations increase the complexity of the overall DSL system. The system performance trade-off introduced by FEC in the form of Reed-Solomon coding and convolutional interleaving can be described as increased data transmission reliability at the expense of increased channel latency.
U.S. Pat. No. 5,764,649 to Tong discloses a system and method compliant with the T1.413 standard. As illustrated in the '649 patent, both a “fast path” and an interleave path are provided downstream of the FEC. As taught in the '649 patent, two frames are output from multiplexer every frame period. One frame is sent through ADSL transmitter along a “fast path” while the other frame is sent along an “interleave path.” The fast path is so called simply because the data does not undergo the additional processing of interleaving, and therefore does not experience the additional delay imposed by de-interleaving at the receiving end of the communication system. However, all data from the incoming bit stream is passed through the FEC, and therefore encounters the latency delay associated therewith.
Accordingly, there is a need to provide an improved system and method for encoding DSL information streams to further minimize latency delays. Further, there is a desire to provide an improved system and method for encoding DSL information streams having differing latencies.